The present invention relates to a spreading code generator, and to a direct-sequence code-division multiple-access communication system using this spreading code generator for both transmitting and receiving at all stations.
Direct-sequence code-division multiple-access (which is properly abbreviated as DS-CDMA, but will be referred to below simply as CDMA) is a type of spread-spectrum communication system in which multiple signal channels occupy the same frequency band, being distinguished by the use of different spreading codes. CDMA communication is employed in, for example, digital cellular telephone systems and personal communication services. In these systems, a base station communicates with a plurality of mobile stations, one frequency band being used for all of the up-links from the mobile stations to the base station, and another frequency band being used for all of the down-links from the base station to the mobile stations.
Conventional CDMA systems have employed different modulation techniques on the up-links and down-links. U.S. Pat. No. 5,103,459, for example, describes a system in which a base station spreads data signals destined for different mobile stations by mutually orthogonal codes, while each mobile station uses all of the orthogonal codes for m-ary encoding of data to be transmitted to the base station. The base and mobile stations also employ pseudorandom noise codes with different periods, and the base station transmits all channels synchronously, signal while the mobile stations transmit asynchronously.
All of the orthogonal codes and pseudorandom codes employed in this system can be described as spreading codes, meaning that they have a higher bit rate or "chip" rate than the bit rate of the transmitted data, but due to the major differences in code structure and usage between the base and mobile stations, each station requires at least two different types of spreading code generators. Moreover, the spreading demodulators at the base station differ in structure from the spreading demodulators at the mobile stations. A resulting disadvantage to the manufacturer is the need to design and manufacture various different types of spreading code generators and demodulating circuits, raising the overall cost of the communication system.